Priority to German Patent Application No. 101 15 875.0, filed Mar. 30, 2001 and hereby incorporated by reference herein, is claimed.
The present invention is directed to an image-recording device for a printing form, including an array of light sources and a downstream microoptics which generates a virtual image of the light sources.
The use of light source arrays in rows or in matrix form for recording images on printing forms, whether in a printing-form exposure unit or in a direct-imaging print unit, places high demands on the imaging optics to be used. Typically, the light source arrays are made up of a specific number of diode lasers, preferably of single-mode lasers, which are mounted at a defined distance from one another, usually spaced apart at substantially the same intervals on a semiconductor substrate, and which share a common exit plane that is precisely defined over the crystallographic plane of fracture. The light-emission cones of these light sources or diode lasers open at different widths in the two planes of symmetry which are substantially orthogonal to one another. From this, the necessity arises of an imaging optics which, on the one hand, reduces, preferably minimizes this asymmetry by using a preferably small number of subassemblies, and, on the other hand, renders possible a global imaging of the array of emitters that is as free of aberrations as possible.
From the related art, one knows of a number of optical imaging systems, which are specially implemented for imaging diode laser arrays to form images on a light-sensitive medium. For example, from U.S. Pat. No. 4,428,647, a semiconductor laser array is known, each of whose individual lasers is assigned an adjacent lens between the laser array and the objective lens. The purpose of these lenses is to change the angle of divergence of the light beams emerging from the surface of the laser array, such that the light is collected as efficiently as possible by the objective lenses and is focused at a light sensitive medium. The optical power of these lenses is selected such that, for each laser, a virtual intermediate image is formed behind the emitting surface, whose spacings correspond approximately to the spacings of the emitted light beams, the emitter""s intermediate image being magnified.
EP 0 694 408 B 1 describes, for example, how a microoptics is able to reduce the divergence of the emerging light by using axially symmetric optical elements.
The often exceptionally large difference in the lateral field dimensions of a light source array of this kind, for example 10xc3x970.001 mm2, therefore requires a specific microscopic and macroscopic image formation. A use of spherical optics for these dimensions can only succeed by employing a relatively large and costly optical design. A disadvantage encountered when using a spherical macrooptics is the variable image quality as a function of the distance to the optical axis. Even the use of cylinder lenses and cylinder lens arrays has, to date, not produced the consistent quality desired for an imaging of a light source array, particularly in the form of a diode laser array.
From U.S. Pat. No. 3,748,015, one knows of an optical system for forming an image of an object with unit magnification and high resolution, which includes an arrangement of a convex and concave spherical mirror, whose centers of curvature coincide at one point. This mirror arrangement produces at least three reflection points within the system and two conjugate regions set apart from the optical axis, at unit magnification in a plane which contains the center of curvature, the optical axis of the system being orthogonal to this plane in the center of curvature. Such a combination of mirrors is free of spherical aberration, coma and distortion, and, when the algebraic sum of the powers or refractive powers of the mirror reflecting surfaces utilized is zero, the image produced is free from third order astigmatism and field curvature. An optical system of this kind is referred to as an optical system of the Offner type.
U.S. Pat. No. 5,592,444, for example, describes a method and a corresponding device for writing and reading data to an optical storage medium, simultaneously in a plurality of tracks. The imaging optics described in this document for a plurality of individually controllable diode lasers includes, in this context, a system of spherical mirrors of the above-described Offner type, thus a combination of spherical concave and convex mirrors having a common center of curvature. However, no virtual, in particular no magnified intermediate image is produced by the divergence-reducing micro optics.
However, the use of an image-recording device for a printing form in a printing-form exposure unit or in a print unit in a printing press requires additional measures. Since, on the one hand, machines of this kind have a very limited assembly space, and, on the other hand, little can be altered on the design or on the configuration of the printing-form exposure unit or on the print unit to implement an image-recording device, it is necessary to reduce the required assembly space. In addition, an imaging optics on a printing press or a printing-form exposure unit is subject to shocks or vibrations, so that it should have as few parts as possible that require relative adjustment. For that reason, known related-art optical systems cannot simply be transferred for use on a printing-form exposure unit or within a print unit of a printing press.
An object of the present invention is, therefore, to devise an imaging optics for an array of light sources, which will reduce the divergence of the emitted light in simple fashion and render possible an image formation having few aberrations. It is additionally or alternatively intended to realize an imaging optics for an image-recording device for a printing form which will require the least possible amount of overall space and as few as possible parts, and therefore, as few as possible degrees of freedom in the adjustment.
The present invention provides an image-recording device for a printing form (29), including an array of light sources (12) and a downstream microoptics (14) which generates a virtual intermediate image (18) of the light sources (12). Arranged downstream from the microoptics is an optical system (10), which includes at least one sector of a convex mirror (26) and one sector of a concave mirror (24) having a common center of curvature, which produces a real image (28).
The image-recording device according to the present invention for a printing form, having an array of light sources and a downstream microoptics which produces a virtual image of the light sources, is distinguished by the microoptics having the downstream optical system, which includes at least one concave mirror sector and one convex mirror sector having a common center of curvature, the algebraic sum of the powers of the refractive powers preferably being zero, in other words, a macrooptics or combination of the Offner type, which produces a real image of the virtual intermediate image. In the following, a convex and concave mirror arrangement is also discussed in simplified terms, although, here as well, at least one mirror may only have one sector that defines a surface that is simply as well as non-simply cohesive, in a specific subspace angular range of maximally 4xcfx80. In this context, one specific embodiment provides, in reality, that the centers of curvature of the concave mirror and of the convex mirror need not coincide with complete precision in order to obtain the desired properties of the Offner-type optical system, exactly enough, for use in an image-recording device according to the present invention.
Using a small number of optically refractive surfaces, in the image-recording device of the present invention, each light source of the array is adapted via a virtual intermediate image to the microscopic requirements, thus, in particular, to the divergence. A downstream macroscopic imaging, utilizing known properties of an Offner-type optical system, thus a combination of at least one convex mirror sector and one concave mirror sector having a common center of curvature, enables points to be advantageously imaged along a line that essentially runs in a circle. The optical system, which, as macrooptics, is positioned downstream from the microoptics, of the image-recording device of the present invention is designed such that the virtual intermediate image points of the light sources, which are essentially arranged in one row, are spaced at a smaller distance to this circular line. In other words: the image-recording device of the present invention makes it possible for the emission from a multiplicity of light sources, in particular from diode lasers, to be constantly corrected using a small number of optical elements. By combining cylindrical lenses, one achieves a micro-optical symmetrization, simultaneously accompanied by magnification, using a virtual intermediate image of each light source and a, to the greatest degree, aberration-free imaging of these virtual intermediate images into a real image, by way of a downstream optical system of a convex mirror and a concave mirror, to create an image-recording device for a printing form having especially beneficial image-forming properties.
To facilitate adaptation of the divergence of the emitted light, the microoptics preferably has an aspherical design. These may be, for example, cylindrical lenses or a combination of anamorphotic prisms. The downstream, macroscopic, optical system of a convex and a concave mirror has at least one circular segment of rotationally symmetric optics, to whose assigned object circle, the essentially straight-line projection of the row of virtual, intermediate image points exhibits a spacing that is kept small, the object circle being situated within one of the two conjugate regions of the optical system of a convex and concave mirror. Thus, using the optical system of the Offner type, the essentially straight-line row of virtual, intermediate image points may be produced as real images, with unit magnification, in the second conjugate region. Especially advantageous in this context is the absence of aberration in the optical system of a convex and a concave mirror.
To reduce the overall space required for the image-recording device of the present invention, the optical path is advantageously folded at least once within the optical system of one convex and one concave mirror. Therefore, at least one path-folding surface is beneficially provided in the optical system situated downstream from the microoptics, whether it be upstream and/or downstream from the reflective surfaces of the optical system of a convex and a concave mirror. This yields a compact optical path through the imaging optics of the image-recording device of the present invention, so that it is possible to reduce the overall required space for an implementation within a printing-form exposure unit or a print unit. Moreover, at least one part of the optical system of a convex mirror and of a concave mirror may be fashioned quite advantageously as a single component, thus monolithically from a suitable material having a refractive index that differs from the ambient environment, for example from a glass or a another transparent material. The individual component, i.e., the monolith may then have partially internally reflecting surfaces, which, for example, form the concave and convex reflective surfaces, respectively, of the optical system of a convex and a concave mirror. These internal surfaces are also described as the active internal surfaces of the monolith. Provided at the monolith are at least one entrance window and one exit window for the light emitted by at least one light source, the windows preferably having an antireflection coating in the form of an interference filter. In one advantageous further embodiment, other optical elements, such as prisms or path-folding surfaces may be assigned to the monolithic structure for purposes of beam deflection.
An image-recording device according to the present invention may be utilized to special advantage in a printing-form exposure unit or in a print unit. A printing press in accordance with the present invention, which includes one feeder, at least one print unit, and a delivery unit, has the distinguishing feature of having at least one print unit equipped with an image-recording device according to the present invention.